Transport Properties Of Compensated Semimetal NbSb₂ And BP/PMN-PT Device With Van Der Waals Heterostructures

Since the ongoing evolution of electronic devices demands for high performance materials,material science and condensed matter physics have developed rapidly in recent years.New generation information materials focus on 1)quantum&topology related insulators,semimetals,2)high performance multiferroic materials/structures,3)low dimensional materials,to achieve faster response,higher efficiency and lower consumption devices.The related researches,including ab-initial calculation,high quality single crystals fabrication,structures/devices'design,transport/photoelectricity characterization,are drawing increasing attention.The presenting thesis aims at exploring new material systems,which will promote technological breakthroughs.In accordance with the most concerned topics,we focus our explorations on the basis of topology,multiferroic and two-dimensional material systems.First,we seek for topological non-trivial new materials from the compensated semimetals with extremely large magnetoresistance effect.Second,we construct a new system of2D/PMN-PT with van der Waals heterostructures,which combines the advantages of two dimensional and ferroelectric materials.By polarizing the ferroelectric substrate,we can tune the physical properties of two dimensional materials,and thus explore their transport,photoelectric,etc.properties.These works will certainly promote the development of new devices with innovation concept.The results are summarized as follows:1)High quality NbSb₂ single crystals have been successfully grown via a new method—chemical vapor transport?CVT?.MPn₂-type?M=Nb,Ta;Pn=P,As,Sb?compensated semimetal NbSb₂ with centrosymmetric C12/m1 space group,paramagnetic ground state,and non-saturation parabolic-like magnetoresistance is considered to be a candidate for topological semimetal.NbSb₂ crystals show metallic conductivity down to 2 K and undergo a metal-to-insulator-like transition with an external magnetic field B?B?4 T?and exhibits a resistivity plateau in the low-temperature region?T?10 K?,in which,the resistivity only depends on the magnitude and direction of the magnetic field.Upon scanning the perpendicular?to current?magnetic field from 0 to 14.5 T at T=1.5 K,NbSb₂ shows a non-saturation parabolic-like magnetoresistance?4.2?10³%at B=14.5 T?with Shubnikov–de Haas?SdH?oscillation.Hall measurements reveal that both the carrier compensation between electrons and holes(The mobility of electron and hole carriers is 5.00?10³ cm²V^-1S^-11 and 4.70?10³ cm²V^-1S^-11 respectively.),which together with the high mobility and large mean free path of carriers,contributes to the large magnetoresistance.Fast Fourier transform?FFT?analyses of angle-resolved SdH oscillation indicate that the Fermi surface of the NbSb₂ crystal is quasi-two-dimension with three-dimensional components.From the absence of negative magnetoresistance effect,and Landau fan diagram,we can roughly deduce NbSb₂'s topological trivial band structure.NbSb₂ may be just only a compensated semimetal with extremely large magnetoresistance effect,and the final determination of its topological properties depends on further theoretical and experimental studies,especially the direct observation of its Fermi surface by angular resolved photoemission spectroscopy?ARPES?.2)A new system of BP/PMN-PT with van der Waals heterostructures was successfully constructed via the tradional mechanical exfoliation method.A series of morphology and structure characterization including XRD,XPS,Raman and AFM were carried out,indicating that the size of the selected black phosphorus is about 5×30 um² and the thickness is 10nm?about 17 layers?.On this basis,we constructed the PMN-PT based new devices?field effect transistor and photodetector?with the help of micro/nano-fabrication technology.The reliability of the device were analyzed by the output and transfer characteristic curve.Linear output characteristic curve indicates a good Ohmic contact between the metal electrodes and the BP flake,while the transfer curve exhibits a typical p-type transport behavior with a high on/off current ratio of 4×10².In addition,the new device shows a good negative gate voltage modulation characteristic.Responsivity,external quantum efficiency and response time are usually used to describe the photoelectric properties of devices.Under the excitation of 1550nm lasber,the new BP/PMN-PT photodetector shows a power-dependent photoelectric response characteristic and its responsivity is 0.697AW^-1as well as the external quantum efficiency is 55.8%.Furthermore,the response and recovery speed can be greatly improved by the negative gate voltage modulation.The incomplete injection of carriers in the polarization process explains the failure of the tune of the electrical and magnetic transport properties.Finally,with the help of optical excitation and ferroelectric polarization,we have developed a new type of storage device with innovation principle.According to the different responsivity of the device under different polarization states,the concept of non-volatile high-density storage is proposed.Polarization and optical responsivity broaden the freedom of storage and provide more storage units according to the difference of its photocurrent and responsivity.It is a meaningful attempt to create new devices with innovation concept.